pointer dereference

[Richard Heathfield]

Re: pointer dereference

santosh said:

Richard Heathfield wrote:

>santosh said:
>>

<snip>

>

* A pointer of type void should be able to hold the highest
possible
address of the program's address space.

>>
>A pointer of type void *, actually. [...]

>
Maybe this is a language issue. Would I be correct when I say "A
pointer to type void"? I don't think so.

Better: "a pointer of type void *"

So I suppose instead of
saying, for example, "a pointer to type int", I should say "a pointer
of type int *" or, better yet, "an int pointer"...?

Either works. Personally I prefer the "a pointer of type <type>" style,
but "a <typepointer" is also correct.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Keith Thompson]

Re: pointer dereference

santosh <santosh.k83@gm ail.comwrites:
[...]

Maybe this is a language issue. Would I be correct when I say "A
pointer to type void"? I don't think so. So I suppose instead of
saying, for example, "a pointer to type int", I should say "a pointer
of type int *" or, better yet, "an int pointer"...?

In my opinion, "a pointer to type void" or "a pointer to type int" is
ok, but "a pointer of type void*" or "a pointer of type int*" is
better. Pointer objects don't point to types, they point to objects.
Pointer types don't point to anything, but objects and values of
pointer types may point to objects. But we can colloquially say that
type int* points to type int; it's not 100% accurate, but it's not
ambiguous, and I wouldn't complain about the usage unless I was
invited to.

I wouldn't use the phrase "an int pointer" because it could imply
something that's both an int and a pointer, rather than something that
points to an int.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

Re: pointer dereference

Chris Dollin <chris.dollin@h p.comwrites:

Christopher Benson-Manica wrote:

>Chris Dollin <chris.dollin@h p.comwrote:
>>

>>I think it's more reasonable to claim that there's exactly one
>>value of type void -- we could call it, oh I dunno, sponk or flolo
>>or `unit` -- which is conveniently represented with zero
>>bits. (Not bits that are zero, of course.)

>>
>It seems like that value could be called "the null value" without too
>much confusion (no more, or less, than with the common term "the
>null character"). That said, I read n869 6.2.5 p18 - "The void type
>comprises an empty set of values..." - as explicitly contradicting the
>notion of there being any values of type void.

>
I think it's more reasonable to claim that there's exactly one value of
type void -- ie, I think that the Standard's statement otherwise is
a less-optimal choice. But this is discussion about the Standard rather
than the language it defines ...

I disagree; I think it would be unreasonable to say that there's a
value of type void.

Why can't you write this?

void *p = /* whatever */
void obj = *p;
void func(void);
obj = func();

If we say that type void has no values, the answer is obvious. If we
say it has one value, we need special-case rules to say that we can't
use that value, and that sizeof(void) is not 0 but a constraint
violation.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Mark McIntyre]

Re: pointer dereference

On Thu, 12 Jul 2007 01:55:13 -0700, in comp.lang.c , somenath
<somenathpal@gm ail.comwrote:

>Hi All,
>I have one doubt regarding pointer .I have one small code as
>mentioned bellow .
>
>#include<stdio .h>
>#include<stdli b.h>
>int main (void)
>{
int *p;
int *ptr= malloc(sizeof(i nt *));
return 0;
>}
>My doubt is if (sizeof(int *)); is undefined ?

sizeof(int*) can't be undefined. It is a constant, available at
compile-time to your compiler from its knowledge of the size of types
on your system.

>Because "p" is point to any where .

p isn't used in your code. You don't need it.



--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

[Richard Bos]

Re: pointer dereference

Chris Dollin <chris.dollin@h p.comwrote:

Christopher Benson-Manica wrote:
>

Chris Dollin <chris.dollin@h p.comwrote:

I think it's more reasonable to claim that there's exactly one value of
type void -- we could call it, oh I dunno, sponk or flolo or `unit` -- which
is conveniently represented with zero bits. (Not bits that are zero, of
course.)

It seems like that value could be called "the null value" without too
much confusion (no more, or less, than with the common term "the
null character"). That said, I read n869 6.2.5 p18 - "The void type
comprises an empty set of values..." - as explicitly contradicting the
notion of there being any values of type void.

>
I think it's more reasonable to claim that there's exactly one value of
type void -- ie, I think that the Standard's statement otherwise is
a less-optimal choice.

I think that's not reasonable at all. void is the empty set, and the
empty set has no members, not even a single, unique, empty member.

Richard

[Chris Dollin]

Re: pointer dereference

Richard Bos wrote:

Chris Dollin <chris.dollin@h p.comwrote:

>I think it's more reasonable to claim that there's exactly one value of
>type void -- ie, I think that the Standard's statement otherwise is
>a less-optimal choice.

>
I think that's not reasonable at all. void is the empty set, and the
empty set has no members, not even a single, unique, empty member.

`void` isn't the empty set. It's a type (name). Types with no elements
are a pain. You may wish to identify the type with the empty set, but
I'd bet money that dollin@hp wouldn't make that identification.

There is, after all, a difference between a function [1] that has
no result, and a function that has a trivial result.

[1] Not necessarily a /C/ function.

--
Far-Fetched Hedgehog
Notmuchhere: http://www.electrichedgehog.net/

[Keith Thompson]

Re: pointer dereference

rlb@hoekstra-uitgeverij.nl (Richard Bos) writes:

Chris Dollin <chris.dollin@h p.comwrote:

>Christopher Benson-Manica wrote:

Chris Dollin <chris.dollin@h p.comwrote:
>
>I think it's more reasonable to claim that there's exactly one
>value of type void -- we could call it, oh I dunno, sponk or
>flolo or `unit` -- which is conveniently represented with zero
>bits. (Not bits that are zero, of course.)
>
It seems like that value could be called "the null value" without too
much confusion (no more, or less, than with the common term "the
null character"). That said, I read n869 6.2.5 p18 - "The void type
comprises an empty set of values..." - as explicitly contradicting the
notion of there being any values of type void.

>>
>I think it's more reasonable to claim that there's exactly one value of
>type void -- ie, I think that the Standard's statement otherwise is
>a less-optimal choice.

>
I think that's not reasonable at all. void is the empty set, and the
empty set has no members, not even a single, unique, empty member.

And if you want a type with a single value:

enum { zero };

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Chris Dollin]

Re: pointer dereference

Keith Thompson wrote:

Chris Dollin <chris.dollin@h p.comwrites:

>I think it's more reasonable to claim that there's exactly one value of
>type void -- ie, I think that the Standard's statement otherwise is
>a less-optimal choice. But this is discussion about the Standard rather
>than the language it defines ...

>
I disagree; I think it would be unreasonable to say that there's a
value of type void.
>
Why can't you write this?
>
void *p = /* whatever */
void obj = *p;
void func(void);
obj = func();
>
If we say that type void has no values, the answer is obvious. If we
say it has one value, we need special-case rules to say that we can't
use that value, and that sizeof(void) is not 0 but a constraint
violation.

Your argument is of the form "if there was a value of type void, then
the following incorrect-C-code would need special-case rules to
explain its incorrectness".

My response is "if there were a value of type void, then that C code
wouldn't need to be illegal, so no special explanation of its illegality
would be required".

When I said:

>I think that the Standard's statement otherwise is a less-optimal choice

I meant that had the Standard chosen void-has-one-value then the Standard
as a whole would have been better (and the language no worse).

--
RIP Donald Michie 11 November 1923 - 7 July 2007

Hewlett-Packard Limited Cain Road, Bracknell, registered no:
registered office: Berks RG12 1HN 690597 England

[Chris Dollin]

Re: pointer dereference

Chris Dollin wrote:

Keith Thompson wrote:
>

>Chris Dollin <chris.dollin@h p.comwrites:

>

>>I think it's more reasonable to claim that there's exactly one value of
>>type void -- ie, I think that the Standard's statement otherwise is
>>a less-optimal choice. But this is discussion about the Standard rather
>>than the language it defines ...

(fx:snip)

When I said:
>

>>I think that the Standard's statement otherwise is a less-optimal choice

>
I meant that had the Standard chosen void-has-one-value then the Standard
as a whole would have been better (and the language no worse).

And when I said

>>But this is discussion about the Standard rather
>>than the language it defines ...

by "the language" I meant to mean C-as-is-currently-defined, ie, that
there was wandering on the edges of, if not outside, topicality. I
didn't mean to imply that the-Standard-with-singleton-void would
define exactly the same language as the-real-Standard.

--
RIP Donald Michie 11 November 1923 - 7 July 2007

Hewlett-Packard Limited registered office: Cain Road, Bracknell,
registered no: 690597 England Berks RG12 1HN

[Army1987]

[OT] Re: pointer dereference

On Fri, 13 Jul 2007 06:39:49 +0000, Chris Dollin wrote:

Richard Bos wrote:
There is, after all, a difference between a function [1] that has
no result, and a function that has a trivial result.
>
[1] Not necessarily a /C/ function.

In the non-C meaning of the word, there is no such thing as a
function with no result, unless it is the empty function (and so
its domain, as well as its codomain, is empty). There is no
function with empty codomain and nonempty domain.
--
Army1987 (Replace "NOSPAM" with "email")
"Never attribute to malice that which can be adequately explained
by stupidity." -- R. J. Hanlon (?)

[Christopher Benson-Manica]

Re: pointer dereference

Chris Dollin <chris.dollin@h p.comwrote:

Keith Thompson wrote:

Chris Dollin <chris.dollin@h p.comwrites:

void *p = /* whatever */
void obj = *p;

Your argument is of the form "if there was a value of type void, then
the following incorrect-C-code would need special-case rules to
explain its incorrectness".

My response is "if there were a value of type void, then that C code
wouldn't need to be illegal, so no special explanation of its illegality
would be required".

I meant that had the Standard chosen void-has-one-value then the Standard
as a whole would have been better (and the language no worse).

I think if you follow Keith's example a bit further, the "language no
worse" argument begins to break down:

void *p=&foo;
void *q=&bar;
void r=*p; /* legal given your arguments */
void s=*q; /* also legal */
if( r == s ) {
/* MUST be true, unless there is more than one value of type void,
but what's the point? */
}

Unless I'm mistaken, void r=*p would require the language to define
conversion from values of all types to this magical single value of
void. Obviously, restoring an object of foo's type from r would be
impossible. The conversion of values of other types to this void type
would also (modulo grains of salt) allow such silly statements as

void t=3; /* ridiculous */

A language that disallows such inherently meaningless constructions
would seem to be better than one that permits them, yes? I certainly
don't see any value gain from introducing a special value of type
void.

--
C. Benson Manica | I *should* know what I'm talking about - if I
cbmanica(at)gma il.com | don't, I need to know. Flames welcome.

[Chris Dollin]

Re: pointer dereference

Christopher Benson-Manica wrote:

(responding to dollin@hp)

I think if you follow Keith's example a bit further, the "language no
worse" argument begins to break down:
>
void *p=&foo;
void *q=&bar;
void r=*p; /* legal given your arguments */
void s=*q; /* also legal */
if( r == s ) {
/* MUST be true, unless there is more than one value of type void,
but what's the point? */
}

What's the point of `if(1) ...`? Of `x = x;`? Of `printf("");`?
It's all the same thing -- trivial cases of general rules.

Unless I'm mistaken, void r=*p would require the language to define
conversion from values of all types to this magical single value of
void.

If it were allowed (it need not be, just as you can't assign a
struct to an int -- that's a separate decision). Yes. So?
The conversion is one of the easier ones to understand ...

Obviously, restoring an object of foo's type from r would be
impossible.

That's right. Just as, after `int x = 12345; char y = x;` you
can't restore the value of `x` from the value of `y`.

The conversion of values of other types to this void type
would also (modulo grains of salt) allow such silly statements as
>
void t=3; /* ridiculous */

It's not /ridiculous/, or /silly/, any more than adding `0` or
multiplying by `1`. It's /trivial/.

A language that disallows such inherently meaningless constructions

It's /not meaningless/. It has a perfectly well-defined and implementable
semantics -- which one, depends on whether conversion to void is
implicit or not.

would seem to be better than one that permits them, yes? I certainly
don't see any value gain from introducing a special value of type
void.

Simplification of the description and use of the language. Most
of the special cases of use of void disappear, or are replaced
by obviously-degenerate cases.

--
Singular Hedgehog
"I just wonder when we're going to have to sit down and re-evaluate
our decision-making paradigm." /Sahara/

[Chris Torek]

Re: pointer dereference

>Chris Dollin <chris.dollin@h p.comwrote:

>I meant that had the Standard chosen void-has-one-value then the Standard
>as a whole would have been better (and the language no worse).

I think so too, although the differene is pretty marginal (in my opinion).

In article <f78250$d28$1@c hessie.cirr.com >,
Christopher Benson-Manica <ataru@vinland. freeshell.orgwr ote:

>I think if you follow Keith's example a bit further, the "language no
>worse" argument begins to break down:
>
>void *p=&foo;
>void *q=&bar;
>void r=*p; /* legal given your arguments */
>void s=*q; /* also legal */
>if( r == s ) {
/* MUST be true, unless there is more than one value of type void,
but what's the point? */
>}

I do not see this as a problem. In a language "like C but with
sizeof(void)==0 and in which the one value of type void, which
occupies no bits, appears to be zero at all times" (which I have
proposed before), we get:

r == s
r == 0
r != 3

Values of type "void" can be used in any integer or boolean context
and simply appear to be zero. Values can be converted to "void",
by cast or assignment; this discards all their bits.

One can also do:

void *p = &foo;
...
p++; /* but afterward, p==&foo */

>Unless I'm mistaken, void r=*p would require the language to define
>conversion from values of all types to this magical single value of
>void.

It is not very "magical", it is just 0. The conversion would be
defined for integers, at least, and maybe for pointers as well (a
la C99's boolean type), but probably not for struct and union.

>... void t=3; /* ridiculous */

And yet, in C as it stands today, we can write:

(void) 3;

which is just as ridiculous. Why can we *cast* to void, but not
*assign* to void?

The main weirdness with variables of type void (in this not-quite-
but-almost-C language) is that, because sizeof(void) is 0, given:

void a;
int b;
void c;
int d;

it is possible to have &a == &b, &a == &c, and/or &a == &d (but
&b != &d, so if &a==&b then &a!=&d). (This same situation occurs
today in GNUC, though, using arrays of size 0.)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Army1987]

Re: pointer dereference

On Fri, 13 Jul 2007 18:10:28 +0000, Chris Torek wrote:

The main weirdness with variables of type void (in this not-quite-
but-almost-C language) is that, because sizeof(void) is 0, given:
>
void a;
int b;
void c;
int d;
>
it is possible to have &a == &b, &a == &c, and/or &a == &d (but
&b != &d, so if &a==&b then &a!=&d). (This same situation occurs
today in GNUC, though, using arrays of size 0.)

And malloc(0) would then make sense.
--
Army1987 (Replace "NOSPAM" with "email")
"Never attribute to malice that which can be adequately explained
by stupidity." -- R. J. Hanlon (?)

[Keith Thompson]

Re: pointer dereference

Chris Dollin <eh@electriched gehog.netwrites :
[...]

Simplification of the description and use of the language. Most
of the special cases of use of void disappear, or are replaced
by obviously-degenerate cases.

None if which, as far as I can see, would be at all useful.

Currently, there are a lot of things you *could* write in a C program
assuming there's a value of type void. Most of them are constraint
violations, emaning that the compiler will tell you about your error.
And they're not special cases; they follow straightforward ly from the
fact that void is an incomplete type. We need rules for incomplete
types anyway, to cover things like 'struct foo' before the full
declaration.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"